The non-equilibrium ro-vibrational distribution functions of molecules in a plasma can heavily influence the discharge operation and the plasma-chemistry. A convenient method for measuring the distribution function is coherent anti-Stokes Raman scattering (CARS). CARS spectra are measured in a ns-pulsed plasma between two parallel, 1 mm spaced molybdenum electrodes in nitrogen at 200 mbar with pulse durations of 200 ns/250 ns and a repetition rate of 1 kHz. The CARS spectra are analyzed by a fitting routine to extract information about the vibrational excitation of the nitrogen molecules in the plasma. It is found that during the discharge the vibrational distribution for $v \lesssim 7$ can be described by a vibrational two-temperature distribution function. Additionally, the electric field is measured by the electric field induced second harmonic generation method during the discharge pulse. It is found to be constant in time after the initial ionization wave with values close to 81 Td for the investigated conditions. During the afterglow between two discharge pulses a more general fitting approach is used to obtain the population differences of two neighboring vibrational states. This allows to capture the more complex vibrational dynamics in that time period. The measurement results are discussed in more detail and compared to simple plasma models in a companion paper Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305205).
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The applied voltage by the DC high voltage generator is 3 or 4 kV with a voltage pulse length of 200-250 ns and a repetition rate of 1 kHz. The measured voltage at the powered electrode and current waveforms are supplied in the dataset in Fig 12.
English (United States)
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The discharge consists of two molybdenum electrodes with a cross-section of 20 × 1 mm placed vertically with a distance of 1 mm. Two glass plates are pressed tightly to the electrodes at the front and the back, yielding a well-confined discharge volume of 20 mm × 1 mm × 1 mm. Nanosecond voltage pulse generated by combining a DC power supply (Heinzinger, LNC 6000-10 neg) with a fast high-voltage switch (Behlke HTS-81) is applied to one electrode with the other one grounded.To protect the switch and increase the discharge stability, the current is limited by a 255 Ohm series resistor between the switch and cathode of the plasma reactor. A delay generator (Stanford Research Systems DG535) with external (synchronized to the flashlamp trigger of the Nd:YAG laser used for CARS) triggering is used to trigger the switch with a repetition rate of f p = 1 kHz and an on-time of 200/250 ns. To allow easy discharge operation, the value of the repetition frequency is chosen within a range determined by the energy input to the system (to limit gas heating) and remaining seed electrons from the previous pulse (to ease discharge ignition). The incoming gas consists of pure nitrogen with a total flow rate of 20 sccm. The pressure in the discharge chamber is monitored by a pressure gauge (Pfeiffer vacuum) and is kept constant at 200 mbar by fine adjusting the mechanical needle valve at the gas outflow.
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pure nitrogen, flow rate: 20 sccm, pressure: 200mbar
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Pump wavelengths: 532 nm and 560 nm Stokes wavelength: 607 nm Laser rep. rate: 20 Hz Phase matching: folded BOXCARS, with 50 cm achromat and final spatial resolution along beam axis about 5 mm. Laser energies: 5 mJ each
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Data and Resources
- Fig. 12, current and voltagecsv
Current and voltage waveforms of the discharge.
- Fig. 13, electric fieldcsv
Electric field data points shown in figure 13.
- Fig. 14, CARS spectra, comparison between one temp. and two temp. distributionscsv
CARS spectra shown in figure 14. Raman shift given in wavenumbers, spectrum...
- Fig. 15, Rhcsv
Rh as defined in the paper.
- Fig. 16, hot vib. temperaturecsv
Hot vibrational temperature as shown in figure 16.
- Fig. 17, vib. energy gaincsv
Vibrational energy gain during the discharge pulse.
- Fig. 19/20, densities of excited statescsv
Densities of vib. excited states (divided by the total gas density) used to...